Concrete and cementitious materials generally are a compromise between the conflicting needs to achieve workability for ease of placing and good compaction, usually attained by adding more water, and to achieve strength by reducing the water content and water:cement ratio and increasing the content of dimensionally stable fillers (aggregate). It is particularly important to have a free flowing material when this has to be spread onto a surface, particularly with thin surface layers. Shrinkage is a fundamental problem with cementitious materials especially on site and is frequently the cause of premature failure and unsatisfactory performance due to self-induced cracking and fracture. Shrinkage occurs mainly in relation to the cement particles, due both to drying and chemical, autogenous shrinkage. To reduce shrinkage requires the inclusion of sufficient quantities of dimensionally stable fillers to reduce the quantity of the cement paste. With conventional concrete it is possible to reduce the shrinkage caused by the liquid cementitious phase by incorporating sufficient larger particles of dimensionally stable aggregate, and because the surface area of this larger Filler is limited the water demand to achieve workability and the water:cement ratio may not be too adversely affected. This certainly cannot be done with thin cementitious coatings (too thin to accommodate such larger particles).
It is generally accepted that cementitious materials must be cured and that the premature loss of water (even by evaporation) should be prevented, and, for this reason, they should not be applied to surfaces having a high suction as this removes the water required for hydration and exacerbates shrinkage cracking. Thus, according to the basic technical reference manual `Properties of Concrete`, edited by Professor Neville, "Loss of water by evaporation From the surface of the concrete or by suction from dry concrete below aggravates the plastic shrinkage and can lead to surface cracking . . . Placing concrete on a dry subgrade should be avoided . . . The object of curing is to keep the concrete saturated, or as near saturated as possible, until the originally water filled space in the fresh cement paste has been filled to the desired extent by the products of hydration of the cement . . . The loss of strength due to inadequate curing is more pronounced in thinner elements."
To overcome the problems of shrinkage cracking with thinner cementitious materials which require reasonable strength, such as flooring screeds, it is common to use semi dry mortar mixes. In these the water content is kept down to a minimum and the material is not saturated with water (i.e. there is no liquid cementitious phase), as in the conventional use of on-site concrete. With the use of such semi dry materials the placing and compaction are difficult and it is not possible to achieve a smooth dense surface finish which is often desirable.
The problems of high shrinkage of thin cementitious materials are discussed in International PCT Publication No WO91/12218, and the means of overcoming these is by placing the materials dry so that the solid particles thereof are touching, and then adding the water for hydration, whereby the initial high shrinkage of wet mixed materials resulting from the loss of water separating particles is avoided.
In this case, it is not possible to achieve a coloured surface as the initial surface application of the water tends to wash the fine materials, cement and colour, off the surface and into the body of the material, nor is a very smooth surface practicable.
Although the foregoing two methods allow a stronger mix to be employed in thinner sections than are possible with conventional concrete, the minimum practical thickness is still limited, in the case of semi dry mixes to about 50 mm and in the case of WO91/12218 to about 20 mm.
Cementitious materials as explained above are not regarded as suitable for thin dense smooth surfaces as this would require a high cementitious and water content and this, particularly when combined with rapid drying, results in cracking and premature failure.
Non-cementitious materials such as epoxy resins can be employed to produce a hard, dense, smooth and coloured surface on floors and similar surfaces, but these are relatively expensive and care is required in their use as they can produce toxic reactions.
Where conventional wet placed concrete is employed, a smooth and if required coloured surface can be achieved but this does involve delay/whilst the material sets. In addition, the surface is generally weaker than the main body of the material due to the settlement of the larger aggregates and the consequent concentration of finer particles on the surface which is made worse by the bleed water which rises and increases the water:cement ratio on the surface. The surface is, of course, generally the most vulnerable to attack and this weak surface mortar phase is frequently the cause of premature failure.